When sha256 ASICs were developed, GPU miners switched from bitcoin to scrypt coins.
When scrypt ASICs were developed, they switched to coins with other algorithms, like x11, x13, scrypt-jane, etc.
IMO, algorithms like x11 and x13 will be remained as "ASIC-resistent" for a long time, and so GPU mining won't really be gone.
There are rumours of FGPA's which are capable of mining x11/x13 plus you have the added chance of botnets